Small giants: Tributaries rescue spatially structured populations from extirpation in a highly fragmented stream

Habitat fragmentation is a pervasive threat to biodiversity. Linearly arranged habitats such as stream networks are particularly vulnerable to fragmentation. As the landscape becomes increasingly human-dominated, conservation values of fragmented habitat patches cannot be overlooked. It is critical to understand the demographic mechanisms of population persistence or extirpation in fragmented patches.
We studied the dynamics of spatially structured populations of two Japanese landlocked salmonids persisting for > 30 years in a headwater stream network that is highly fragmented due to low-head dams in the mainstem. We parameterised and analysed spatial matrix population models using 9-year mark-recapture data.
Tributaries supported higher survival rates in some life stages, and movement was asymmetrical from the tributaries to the mainstem. Accordingly, population growth rates were higher in the tributary patches than the mainstem in both species despite the tributaries occupying only...